To the Editor:—
I read with interest the article by Naruo et al. entitled “Sevoflurane Blocks Cholinergic Synaptic Transmission Postsynaptically but Does Not Affect Short-term Potentiation.” I agree with the authors about the importance of studies at the cellular and molecular levels, which in conjunction with studies at the cognitive science level should provide a comprehensive account of effects of anesthetics on memory. However, my enthusiasm for the authors' excellent work was marred by the following points. It is not clear how the neurons used in the cell culture are relevant to memory processes. For example, in a similar aquatic invertebrate, Aplysia , the sensory and motor neurons of the gill withdrawal reflex are commonly used. The reflex in the intact animal can be classically conditioned and undergoes habituation and sensitization. Does this occur with the neurons used in this report?
It is inaccurate to state that numerous studies found no effect of anesthesia on various types of memories. Anesthetics (in anesthetizing dosages) abolish both short- and long-term memories. The authors cite as a reference, an article that was presented at a symposium in 1995 where the authors sent a questionnaire to a number of consultants regarding their opinions about the existence of implicit learning and memory during anesthesia. However, such existence remains controversial, as best exemplified by the lack of replication of any positive findings (except for one recent work by Deeprose et al. ).5,,6
Although it is true that the anesthetized brain is able to process auditory information, this does not allow cognitive processing during adequate anesthesia. Looking at the auditory evoked responses, the brainstem response is resistant to anesthetic effects. The early or midlatency responses that reflect neural transmission through the medical geniculate body in the thalamus to the primary auditory cortex disappear with deep anesthesia. The late cortical responses that reflect transmission through cortical association areas, the frontal cortex and the hippocampus, and are engaged in cognitive processes are abolished with loss of consciousness. The authors give as a reference for persistence of cognitive processing during anesthesia a meta-analysis of studies of implicit memory before 1996. More studies have been published since then, and as with any meta-analysis, the results are dependent on the quality of the reviewed articles. Finally, the authors state that cellular studies are important in resolving the issue of whether anesthetics affect learning and memory. Perhaps it would be more productive for investigators to start with the forgone conclusion that anesthetics do affect learning and memory and to elucidate the sites and mechanisms of this important interaction.
University of Iowa Hospitals and Clinics, Iowa City, Iowa. firstname.lastname@example.org